Mantle Petrology and Geochemistry Beneath the Nógrád-Gömör Volcanic Field, Carpathian-Pannonian Region

Abstract

Neogene to Quaternary alkali basalts within the Nógrád-Gömör Volcanic Field (NGYF) contain abundant mafic and ultramafic xenoliths and megacrysts, yielding insight into the nature and evolution of the mantle beneath this region. The Cr-diopside suite of ultramafic nodules has been classified by texture, mineralogy, and chemistry into two groups: Group 1—spinel lherzolite and spinel websterite, with protogranular to porphyroclastic or equigranular textures and small amounts of pargasitic amphibole; and Group 2—dunite and spinel lherzolite, with “secondary” recrystallized textures and minute amounts of phlogopite. Cr-diopside-rich veinlets occur in Groups 1 and 2, forming composite xenoliths. Calculated bulk-rock compositions and most mineral compositions vary distinctly with texture. “Secondary” recrystallized xenoliths (Group 2) are depleted in “basaltic” major elements, which decrease gradually through equigranular-textured xenoliths towards protogranular to porphyroclastic xenoliths. Nevertheless, within Group 1, amphibole-bearing equigranular and protogranular-to-porphyroclastic xenoliths cannot be distinguished by rare-earth element (REE) contents. Their unfractionated REE patterns and the formation of amphibole can be attributed to modal metasomatism. However, anhydrous xenoliths from Group 1, showing a negative correlation between “basaltic” major and rare-earth elements, are indicative of the effects of cryptic metasomatism. This might be controlled by percolation of metasomatizing melts (Navon and Stolper, 1987; Bodinier et al., 1990) or by a small amount of metasomatizing agents near the vein conduit (Nielson et al., 1993). Principal features of Group 2 xenoliths, such as “secondary” recrystallized textures, clinopyroxenite veins, high-F-content phlogopite, and depletion in “basaltic” major elements, reflect complex mantle events, including modal and cryptic metasomatism. Although the lithospheric mantle beneath the NGVF has been considerably modified, the nature of this upper mantle is consistent with existence of a subducted slab. This subducted slab was the possible source of the metasomatic agents.